Some switching circuits, such as switching regulators for light emitting device (LED) drivers, need to accept power from a variety of voltage sources that may provide a wide range of input voltages—from low voltages produced by DC sources to high voltages supplied from rectified AC lines. In addition, switching circuits may be subject to load uncertainty. For example, LEDs inherently have highly variable I-V characteristics. The variable number of LEDs that may be driven by a LED driver exaggerates the load uncertainty.
Since the light emission of a LED is directly related to current flowing through the LED, the output current of a LED driver should be accurately controlled. As the output current of the LED driver depends on the input voltage, variations in the input voltage level create substantial problems for accurate output current control.
An isolated switching regulator, such as a flyback regulator, is used for LED drivers to accommodate a wide range of input voltages and load uncertainty. Moreover, the isolating arrangement of the flyback switching regulator separates an input voltage source from a load, providing additional safety protection.
However, the isolating arrangement also makes it more difficult to control the switching regulator since the control information should be relayed through the isolating barrier between the input and output sides of the regulator. Moreover, an opto-coupler creates a propagation delay in a feedback loop of a LED driver reducing system dynamics and accuracy.
Therefore, it would be desirable to provide control circuitry and methodology for controlling output current in an isolated switching regulator without directly measuring the current or voltage at the output side of the regulator.